Method for treating a superhard composite material intended for being used in the production of cutting tools

ABSTRACT

The invention relates to a method of processing a superhard composite material (21) comprising a polycrystalline microstructure and a binder, said method comprising the following steps:contacting (200) a surface of said superhard composite material (21) with an absorbent material (30), andapplying (300) an electric current to the superhard composite material (21), causing the binder to move from the superhard composite material (21) to the absorbent material (30) so as to create a continuous gradient (221) of binder content within the superhard composite material (21).

The invention relates to the field of superhard composite materials and more particularly to superhard composite materials that can be used in particular in cutting, machining, or drilling tools. The invention relates to a method of manufacturing such superhard composite materials, the superhard composite materials obtainable by such a manufacturing method, as well as cutting, machining, or drilling tools including such superhard composite materials.

PRIOR ART

Superhard composite materials available on the market, especially cermets (name given to Ceramic-Metal materials), generally comprise a polycrystalline microstructure, the grain joints of which are filled with a binder. These materials have high mechanical strength and are generally used as elements for cutting, machining, or drilling tools. Such use subjects them to severe temperature and pressure conditions. Indeed, the impacts on the work surface during cutting, machining, or drilling processes generate a significant increase in temperature and pressure. Typically, the temperature oscillates around 500° C., but can rise in some applications up to a value greater than 900° C. While pressure resistance depends on the mechanical properties of the superhard composite material, temperature can damage the material by chemical transformation of the polycrystalline microstructure. Damage to the superhard composite material can also be amplified in the event of poor thermal conductivity, with heat being poorly dissipated. Indeed, with use at high temperature, differences in thermal expansion coefficients between the binder and the polycrystalline microstructure lead to the formation of cracks within the composite material, mainly due to the excessive binder content in the grain joints and can also cause a phenomenon of grain loosening.

There are methods in the literature aiming to reduce the concentration of binder at the cutting surface (for example US6410085) of the superhard composite material, and thus to create a binder-depleted area. It is known that such a reduction in binder concentration leads to an improvement in the mechanical and thermal properties of the composite material such as good elasticity and excellent impact resistance. For example, document US6410085 describes a superhard composite material with a polycrystalline diamond microstructure, the grain joints of which in a first volume near the cutting surface are substantially free of binder and, in a second volume, are filled with a binder. However, such a method does not allow significant depths (in the range of 100 µm to 200 µm) to be reached within the superhard composite material.

Among the methods aiming to create a binder-depleted area, it is possible to mention:

-   the chemical attack aiming to leach the binder. Such a method of     depleting by chemical attack is described more particularly in     document US20120152064. This attack consists in using strong acids     (for example hydrofluoric acid, nitric acid, or sulfuric acid) to     leach the binder onto a thin layer of the cutting tool adjacent to     the work surface. This technique results in the formation of a     superhard composite material having two areas with different binder     content: a first area with a nominal content and a second area with     a low content. However, this method has the disadvantages, on the     one hand, of creating an abrupt change in binder concentration     within the material, promoting the occurrence of breaks, and, on the     other hand, of generating waste harmful to the environment. -   Another method for removing the binder consists in using a     high-power laser to remove the metallic binder from the surface of a     polycrystalline microstructure. Although this technique has a short     processing time, the high temperature generated by the laser may     cause damage to superhard composite materials comprising a diamond,     tungsten carbide, or boron nitride polycrystalline microstructure. -   Preparing a functionally graded cemented tungsten carbide material     by heat treatment in a cemented gas environment was proposed in     patent application US20110116963. This document proposes to heat     process cemented tungsten carbide sintered at high temperatures.     Liquid cobalt migrates while the carbon is absorbed by the heated     cermet. The binder content is directly influenced by carbon     absorption and has shown binder depletion on the surface of the     material. However, binder depletion requires very high temperatures     and the gradients are not always continuous over a significant     depth. -   Methods of manufacturing a superhard composite material having a     binder gradient from the infiltration of a cermet-like material have     also been proposed (EP1975264 and Hulbert et al., 2008, Materials     Science and Engineering: A, Volume 488, Issues 1-2, 15 Aug. 2008,     Pages 333-338). Infiltration increases the content of the cermet in     the binder phase and this phenomenon may continue until there is     saturation in the binder phase of the cermet. Nevertheless, this     infiltration is generally done at the melting temperature of the     binder and can lead to material degradation. In addition, it can     lead to the formation of areas with high binder concentrations     weakening the material.

Thus, existing methods have several disadvantages such as, for example, the need for high temperatures, which some polycrystalline microstructures cannot withstand, or the use of polluting substances. In addition, most of these methods do not allow the creation of a continuous gradient of binder content over a significant depth.

Thus, there is a need for new methods of manufacturing superhard composite materials capable of addressing the issues caused by existing methods.

TECHNICAL PROBLEM

The invention therefore aims to overcome the disadvantages of the prior art. In particular, the invention aims to propose a new method of processing a superhard composite material, with said method not producing toxic waste, allowing recycling of the binder, not requiring an excessive increase in temperature, typically below 1200° C., and allowing to significantly reduce the processing time compared to conventional methods such as chemical attack.

The invention also aims to provide a new superhard composite material, with said superhard composite material having improved thermal and mechanical properties (such as, for example, improved wear or erosion resistance, good elasticity, and very good impact resistance).

BRIEF DESCRIPTION OF THE INVENTION

To this end, the invention relates to a method of processing a superhard composite material comprising a polycrystalline microstructure and a binder, said method comprising the following steps:

-   contacting a surface of said superhard composite material with an     absorbent material, and -   applying an electric current to the superhard composite material,     causing the binder to move from the superhard composite material to     the absorbent material so as to create a continuous gradient of     binder content within the superhard composite material.

Implementing this manufacturing method is fast and does not generate toxic substances. On the contrary, using an absorbent material allows to temporarily capture some of the binder present in the superhard composite material, which can then be recycled. In addition, implementing this manufacturing method allows to obtain a binder-depleted area with a continuous gradient of binder content over a significant depth. This binder-depleted area is obtained by implementing electromigration. Electromigration is a phenomenon generally observed in microelectronics and corresponds to the migration of metal ions under the effect of a significant displacement of electrons following the application of an electric current. Here, the application of an electric current for displacing the atoms constituting the binder in its solid form (and not in its liquid form, which would require too high a temperature) allows to binder deplete some of the superhard composite material.

According to other optional characteristics of the method:

-   the polycrystalline microstructure includes crystals selected from     the crystals of: tungsten carbide, diamond, silicon carbide, silicon     nitride, and boron nitride. Preferably, the polycrystalline     microstructure comprises, preferably consists essentially of,     tungsten carbide crystals.

-   the binder includes at least one element selected from the following     elements: iron, ruthenium, osmium, hassium, cobalt, rhodium,     iridium, meitnerium, nickel, palladium, platinum, darmstadtium,     molybdenum, and titanium. Preferably, the binder includes cobalt.     For example, it can be used in combination with other chemical     elements such as iron or nickel. More preferably, the binder     essentially consists of cobalt. -   the absorbent material is capable of interacting with the binder so     as to form binary, ternary, or quaternary compounds. Thus, the     absorbent material will prevent binder build-up on the surface of     the superhard composite material. -   the absorbent material is provided on a surface adjacent to an area     to be binder-depleted. -   the electric current applied to the superhard composite material has     an electric current density between 0.8 and 20 A/mm². -   the electric current is applied over a period of at least five     minutes. -   the electric current is applied to the superhard composite material     via electrodes positioned, on the one hand, on a surface adjacent to     an area to be binder-depleted and, on the other hand, to a surface     opposite to the area to be binder-depleted. -   The processing method is carried out at a temperature below the     melting temperature of the binder. -   The processing method is carried out in a device generating an     electric current at the terminals of two electrodes between which     the material to be treated is placed, such as, for example, a Spark     Plasma Sintering (SPS) device.

The present invention allows to obtain functionally superhard materials with improved mechanical and thermal properties, for example for cutting purposes. These improved properties are achieved by creating in the composite material a binder-depleted area with a continuous gradient of binder content. Since the gradient of binder content is continuous over a significant depth, for example 500 µm or more, this allows to reduce the risk of delamination and flaking of the layers.

Thus, the invention also relates to a superhard composite material with a binder-depleted area, characterized in that the binder-depleted area has a continuous gradient of binder content extending over a depth greater than or equal to 500 µm.

The invention also relates to a cutting, machining, or drilling tool comprising the superhard composite material according to the invention. In particular, the tools according to the invention have a longer service life than traditional tools.

Other advantages and characteristics of the invention will appear upon reading the following description given by way of illustrative and non-limiting example, with reference to the appended figures which represent:

-   FIG. 1 , a schematic representation of the manufacturing method     according to an embodiment of the invention. The steps in dotted     lines are optional. -   FIG. 2 , a cross-sectional view of a superhard composite material on     which electrodes and an absorbent material are positioned according     to an embodiment of the invention. FIG. 2A schematizes the     arrangement before applying an electric current while FIG. 2B     schematizes the arrangement after applying an electric current     according to the invention and in particular the superhard composite     material with a continuous gradient of binder content. The lighter     area in the superhard composite material corresponds to a lower     binder content. -   FIGS. 3A to 3C, curves of the binder content as a function of the     depth from the cutting surface for a polycrystalline diamond -     cobalt superhard composite material with a continuous gradient of     binder content according to the invention. The absorbent material     used in the context of the method according to the invention being     (FIG. 3A) a niobium plate, (FIG. 3B) a niobium powder, (FIG. 3C) a     copper plate, respectively. -   FIG. 4 , a curve of the binder content as a function of the depth     from the cutting surface for a tungsten carbide - cobalt superhard     composite material with a continuous gradient of binder content     according to the invention. The absorbent material used being a     niobium plate. The dotted line corresponds to a linear regression     line and it has in this figure a slope of 0.1 m.mm⁻¹%.

[DESCRIPTION OF THE INVENTION]

In the following description, by “superhard composite material” is meant a poorly compressible material comprising an assembly of at least two immiscible components. Preferably, this so-called superhard material has at least one area with a Vickers hardness of 20 gigapascals or more, and more preferably 40 gigapascals or more. The method for measuring the Vickers hardness is well known to the one skilled in the art and it can be measured, for example, by the standard method EN ISO 6507-1. Due to its hardness, this material is of great interest for many industrial applications, including abrasives, cutting and polishing tools, or protective coatings. A superhard composite material may, for example, be a Cermet (Ceramic-Metal) material comprising an assembly of a polycrystalline ceramic microstructure and a metallic binder.

By “polycrystalline microstructure” is meant a structure or array composed of a plurality of grains or crystals of varying size and orientation, partially interconnected. Preferably, regarding the polycrystalline microstructure according to the invention, the crystals have a nanometric or micrometric size, for example between 50 nm and 100 µm.

By “cermet material” is meant a composite material composed of a polycrystalline ceramic microstructure, the grain joints of which are filled with a metallic binder. The ceramic microstructure can be composed of various elements such as oxides, carbides, nitrides, or borides. For example, the polycrystalline ceramic microstructure may correspond to a microstructure including magnesium oxide, beryllium oxide, aluminum oxide, tungsten carbide, titanium carbide, or titanium boride crystals or grains.

By “binder” is meant one or more compounds that are associated with the polycrystalline microstructure so as to improve the mechanical characteristics of the composite material. In the superhard composite material according to the invention, the binder is preferentially provided in the interstices of the polycrystalline microstructure, that is to say at the level of the grain joints of this microstructure. This binder generally allows to reduce the fragility of the composite material and its sensitivity to cracking, and increase the strength and toughness of the composite material. The binder can correspond to a chemical element, a chemical compound with several chemical elements, or several chemical compounds.

By “absorbent material” is meant a material capable of collecting the binder from the superhard composite material. This is achieved by the interaction of the chemical elements or the chemical compounds included in the absorbent material with the binder. This absorbent material is essential to prevent binder build-up on the surface of the superhard composite material.

By “binder-depleted area” is meant a part or volume of the superhard composite material with a reduced binder concentration compared to the initial concentration (or nominal concentration). The binder-depleted area according to the invention has a continuous gradient of binder content. Similarly, the “area to be binder-depleted” corresponds to a part or volume of the superhard composite material capable of exhibiting, following the implementation of the method according to the invention, a continuous gradient of binder content.

By “cutting surface” is meant a surface of the binder-depleted area that is appropriate for use in a cutting, machining, or drilling tool as a cutting surface because it has improved mechanical and thermal properties, including a high Vickers hardness, for example greater than 40 gigapascals. These improvements are linked in particular to a very low concentration of binder. For example, the concentration of binder in the cutting surface is lower than or equal to 3 wt.%, preferably lower than or equal to 2 wt.%.

By “continuous gradient of binder content” is meant a continuous, preferably linear, change in binder concentration within an area or part of the composite material. That is to say that the change in binder concentration is not in successive steps. For example, a continuous gradient of binder content may have a linear correlation coefficient between the depth and the binder content greater than 0.7, preferably greater than 0.8, and this for example over a distance greater than 200 µm, preferably greater than 500 µm, and even more preferably greater than 1 mm. The continuous gradient of binder content can also be defined by its slope corresponding to the degree of variation of the binder content per unit distance. In the context of the invention, the continuous gradient of binder content is positioned between the cutting surface, an area with a very low binder concentration, and the area with a nominal binder concentration.

By “adjacent” is meant a separation of less than 10 mm, preferably less than 5 mm, and even more preferably a contact between the two objects considered.

B “essentially consisting of” is meant a concentration greater than 90 wt.%, preferably greater than 95%, and more preferably greater than 99%.

By “substantially constant” is meant a value varying from less than 20%, preferably less than 10%, compared to the average value on the area considered.

In the following description, the same references are used to designate the same elements.

FIG. 1 represents the essential and optional steps of an embodiment of the processing method according to the invention.

The processing method according to the invention allow to obtain a superhard composite material with a continuous gradient 221 of binder content. This superhard composite material with a continuous gradient 221 of binder content is particularly advantageous because it has improved mechanical and thermal properties.

The processing method according to the invention requires a superhard composite material 21. The superhard composite material 21 comprises a polycrystalline microstructure and a binder. More particularly, the crystals or grains of the polycrystalline microstructure form an interstitial network containing the binder.

The polycrystalline microstructure can consist of a wide variety of crystals. For example, without being limited to this particular list, the polycrystalline microstructure may include crystals selected from the crystals of: tungsten carbide, diamond, silicon carbide, boron carbide, titanium carbide, tantalum carbide, silicon nitride, boron nitride (especially cubic boron nitride), rhenium boride, tungsten boride, ruthenium boride, iridium boride, boron oxide, alumina, silicon nitride, titanium nitride, aluminum nitride, boron phosphide. Preferably, the polycrystalline microstructure includes crystals selected from the crystals of: tungsten carbide, diamond, silicon carbide, silicon nitride, and boron nitride (especially cubic boron nitride). More preferably, the polycrystalline microstructure includes tungsten carbide crystals. Even more preferably, the polycrystalline microstructure consists essentially of tungsten carbide crystals.

In particular, the polycrystalline microstructure comprises diamond, tungsten carbide, or boron nitride (especially cubic boron nitride) crystals. Indeed, these crystals may not withstand the methods of the prior art using high temperatures. More specifically, the polycrystalline microstructure comprises diamond, or boron nitride (especially cubic boron nitride) crystals. Indeed, these crystals are more sensitive to high temperatures than tungsten carbide crystals, and thus fully benefit from the possibility of carrying out the method according to the invention at low temperatures (for example below 700° C.).

Although the polycrystalline microstructure is preferably formed of a single species of crystals, it is possible to implement the invention on a superhard composite material with a polycrystalline microstructure consisting of a mixture of crystals of different composition.

The presence of the binder generally reduces the brittleness of the composite material and its sensitivity to cracking. In addition, it increases the strength and toughness of the composite material. It is able to be placed preferentially into the interstices of the polycrystalline microstructure and improve the mechanical characteristics of the superhard composite material.

Advantageously, the binder includes metallic elements. For example, the binder may include at least one element selected from the following elements: iron, ruthenium, osmium, hassium, cobalt, rhodium, iridium, meitnerium, nickel, palladium, platinum, darmstadtium, molybdenum, and titanium.

Preferably, the binder includes at least one element selected from the following elements: cobalt, nickel, iron, and molybdenum. More preferably, the binder includes cobalt, for example in combination with other chemical elements. Even more preferably, the binder consists essentially of cobalt.

Although the binder preferably consists of a single metallic element, it is possible to implement the invention on a superhard composite material including a binder consisting of a mixture of different chemical elements, particularly metallic elements.

The superhard composite material 21 is preferably a Cermet (Ceramic-Metal)-like material. In this case, it comprises more particularly a polycrystalline ceramic microstructure and a metallic binder.

In addition to the binder and the polycrystalline microstructure, the superhard composite material 21 according to the invention may include a compound capable of acting as a reinforcement. This reinforcement can be selected for example from fibers, filaments, or particles. For example, the superhard composite material 21 according to the invention may include carbon nanotubes.

In general, the superhard composite material 21 is composed mainly of its polycrystalline microstructure. Thus, preferably, the polycrystalline microstructure represents a content greater than 50 wt.% of the superhard composite material. More preferably, the superhard composite material 21 has between 50 wt.% and 98 wt.% of polycrystalline microstructure, and even more preferably between 70% and 95%. The several elements composing the superhard composite material 21 can be quantified by the methods known to the one skilled in the art such as microscopic analyses or fluorescence.

The superhard composite material 21 may include between 2 wt.% and 40 wt.% of the binder (namely, mass fraction). For example, when the polycrystalline microstructure includes a mixture of crystals, the percentage of binder can be more than 30%. Preferably, the superhard composite material 21 has between 2 wt.% and 30 wt.% of binder. More preferably, the superhard composite material 21 includes between 3 wt.% and 14 wt.% of binder. The mass fraction binder content is measured, for example, by wavelength-dispersive spectrometry.

The superhard composite material 21 described above and used as a starting material in the processing method according to the invention can be obtained by different methods described in the prior art (see Kanyanta, Microstructure-Property Correlations for Hard, Superhard, and Ultrahard Materials, 2016). Among these methods, the preferred method is sintering. Sintering corresponds to a process of densifying and consolidating a powder shaped under the effect of temperature and resulting in a binding of the grains of the polycrystalline microstructure and an increase in the contact interfaces between the grains or crystals due to the movement of atoms within and between the grains of the polycrystalline microstructure. Sintering allows the consolidation of powdery materials at temperatures below their melting points. Thus, the method according to the invention may include a step 100 of preparing (PREP) a superhard composite material 21 by sintering.

Several sintering methods have been developed, including flash sintering. During flash sintering, a powder is compressed uniaxially and quickly heated by direct current pulses through the powder or material. Thus, in the context of the step 100 of preparing the superhard composite material 21, the superhard composite material 21 is preferably prepared by flash sintering. Indeed, this method allows to obtain homogeneously densified materials, and a fast execution (a few minutes instead of a few hours by natural sintering).

In a second step 200 (CONTACT), the superhard composite material 21 is contacted with an absorbent material 30.

A possible formation of binder aggregates was observed on the surface of the superhard composite material 21 when implementing a method similar to that of the invention, but in the absence of an absorbent material. This aggregate formation occurs at the surface through which the electrical current exits the superhard composite material 21. In order to avoid this formation of aggregates that can degrade the properties of the superhard composite material, an absorbent material 30 is used, placed in contact with the superhard composite material and comprising chemical elements likely to combine with the binder. Preferably, the absorbent material 30 includes one or more elements capable of interacting with one or more elements of the binder so as to form new chemical compounds such as binary, ternary, or quaternary compounds. Thus, the presence of the absorbent material 30 allows to prevent binder build-up at one of the ends of the superhard composite material 21.

The absorbent material 30 is provided on a surface adjacent to an area to be binder-depleted. More particularly, the absorbent material 30 is contacted with the area to be binder-depleted, that is to say the area that will comprise, after implementing the processing method according to the invention, the continuous gradient 221 of binder content.

The absorbent material 30 may include at least one element selected from the following elements: scandium, yttrium, titanium, zirconium, hafnium, rutherfordium, vanadium, niobium, tantalum, dubnium, chromium, molybdenum, tungsten, seaborgium, manganese, technetium, thenium, bohrium, iron, ruthenium, osmium, hassium, cobalt, rhodium, iridium, meitnerium, nickel, palladium, platinum, darmstadtium, copper, silver, gold, roentgenium, zinc, cadmium, mercury, copernicium, boron, aluminum, gallium, indium, thallium, carbon, silicon, germanium, tin, lead, nitrogen, phosphorus, arsenic, antimony, and bismuth. Preferably, the absorbent material 30 includes at least one chemical element selected from the following chemical elements: niobium, copper, molybdenum, tantalum, titanium, zinc, aluminum, and iron.

Advantageously, the binder includes cobalt and the absorbent material 30 includes at least one chemical element selected from the following chemical elements: niobium, copper, molybdenum, tantalum, titanium, zinc, aluminum, and iron.

The absorbent material 30 can take different forms. The absorbent material 30 can for example be used in solid form, for example a plate, or in powder form. Indeed, as shown in the examples, for polycrystalline diamond (PCD) a niobium plate is more effective than niobium powder. Thus, advantageously, the absorbent material 30 is used in the form of a plate, wherein said plate may preferably have a thickness greater than 50 µm, for example a thickness between 0.1 mm and 10 mm. Nevertheless, preferably, the absorbent material 30 is used in the form of a powder because it can adapt to the surface of the superhard composite material, for example to the cutting surface, and this therefore allows to process superhard composite materials with complex shapes.

The method according to the invention then includes a step 300 (CRT) of applying an electric current to the superhard composite material 21, causing the binder to migrate from the superhard composite material 21 to the absorbent material 30 so as to create a continuous gradient 221 of binder content within the superhard composite material 21.

The electric current is preferably a continuous electric current. Indeed, a symmetrical alternating current will have little influence on the displacement of the binder.

Preferably, the electric current is not applied in the form of short pulses but is implemented over a period of at least five minutes. Preferably, the electric current is implemented over a period greater than or equal to 5 minutes, more preferably over a period greater than or equal to 30 minutes, even more preferably over a period greater than or equal to 60 minutes. For example, the electric current can be implemented over a period between five minutes and twenty-four hours.

The difference in potential applied at the superhard composite material has little influence on the method according to the invention. It is the current intensity and more particularly the current density that has a strong influence on the efficiency of the method. Thus, preferably, the electric current passing through the superhard composite material 21 has a current density between 0.1 and 50 A/mm², even more preferably it is between 0.4 and 25 A/mm².

The electric current can be implemented through the superhard composite material by various means. For example, the electric current applied to the superhard composite material 21 may be a current induced by an electric field. Applying the electric current can also be carried out using a generator coupled to electrodes and capable of delivering a direct current with an intensity equal at least to 500 A and a voltage of at least 2 V.

The electric current can be applied to the superhard composite material 21 via electrodes, especially with an anode 301 positioned at a surface adjacent to the area to be binder-depleted, for example, on an absorbent material 30 in contact with the superhard composite material 21.

Preferably, the electric current is applied to the superhard composite material 21 via electrodes positioned, on the one hand, at a surface adjacent to an area to be binder-depleted and, on the other hand, at a surface opposite the area to be binder-depleted.

As shown in FIG. 2 , more particularly, the anode 301 is positioned at a surface adjacent to the area to be depleted while the cathode 302 is positioned at a surface opposite the area to be depleted.

More preferably, the anode 301 is in contact with the absorbent material 30, the latter being itself positioned in contact with the area to be binder-depleted so as to form there the continuous gradient 221 of binder content.

Applying this electric current can lead to an increase in temperature within the superhard composite material 21. Advantageously, such an increase in temperature allows to facilitate the migration of the chemical elements of the binder and thus to increase the efficiency of the method according to the invention. Thus, advantageously, the method according to the invention is carried out at a temperature greater than or equal to 300° C. For example, the intensity of the applied electrical current can be advantageously adjusted so that the temperature within the superhard composite material is a temperature greater than or equal to 300° C.

However, the temperature should not be too high either. Indeed, it has been observed that the method according to the invention is more efficient when the temperature of the superhard composite material is kept lower than the melting temperature of the binder. Thus, preferably, the method according to the invention is carried out at a temperature lower than the melting temperature of the binder. For example, the intensity of the applied electric current is advantageously adjusted so that the temperature within the superhard composite material is lower than the melting temperature of the binder.

Similarly, since too high a temperature within the superhard composite material 21 could damage the polycrystalline microstructure, the method according to the invention is preferably carried out at a temperature below 1500° C., more preferably below 1000° C. This is particularly advantageous because some polycrystalline microstructures are sensitive to too high temperatures. For example, a diamond, tungsten carbide, or boron nitride polycrystalline microstructure may be damaged if exposed to excessive temperatures. Thus, preferably, the method according to the invention is carried out at a temperature below 700° C. when the polycrystalline microstructure includes diamond or boron nitride (especially cubic boron nitride).

Thus, advantageously, in a step 400 of the processing method, the temperature of the superhard composite material is controlled so that the temperature is sufficient, that is to say higher than a low threshold temperature T_(SB), for observing electromigration, and sufficiently low, that is to say lower than a high threshold temperature T_(SH), for avoiding degradation of the polycrystalline microstructure. The threshold temperatures are dependent on the elements constituting the superhard composite material 21. For example, for a tungsten carbide and cobalt superhard composite material 21, T_(SB) can be equal to 300° C. and T_(SH) to 1200° C., preferably 1150° C., while for a polycrystalline diamond and cobalt superhard composite material 21, T_(SB) can be equal to 300° C. and T_(SH) to 700° C.

The measured temperature is preferably the temperature of the superhard composite material 21. More preferably, it is the temperature measured at a surface of the superhard composite material 21, preferably adjacent to the area to be binder depleted. This temperature can be measured, for example, using a thermocouple at low temperatures (for example up to 1200° C.), or by an optical pyrometer at high temperatures (for example from 600° C. up to 2000° C.).

Within the method according to the invention, the temperature of the superhard composite material 21 can be modulated via the regulation of an external heating source and/or via the electrical power (for example electric current intensity) applied to the superhard composite material 21. Preferably, the electric current intensity is adjusted so that the temperature is between a determined low threshold temperature T_(SB) and a determined high threshold temperature T_(SH). This is for example shown in FIG. 1 where, if the temperature controlled in step 400 is not between T_(SB) and T_(SH) (N), then the method returns to step 300 so as to modify the applied electrical power (for example electric current intensity) until the temperature is between the desired T_(SB) and T_(SH) limits (O).

The electrical power that can be applied to the superhard composite material can be between 200 W and 2000 W.

The method according to the invention may, preferably, be implemented under a controlled atmosphere adapted depending on the nature of the processed materials. For example, the method according to the invention can be implemented in a resistance furnace, an induction furnace, or a microwave oven, under a protective atmosphere and/or under vacuum. The protective atmosphere is for example argon, a mixture of argon and hydrogen, nitrogen, hydrogenated nitrogen, or hydrogen. The expression “under vacuum” can refer to a primary (~1 Pa) or secondary (~10⁻² Pa) vacuum.

The method according to the invention may also include a step 500 (VAR) of electrical power, temperature, and/or pressure cycles. Such cycles are defined by increasing rates, leveling times, and decreasing rates.

Thus, in particular, the method according to the invention comprises at least two cycles of electric current, with said cycle of electric current including an increase and a decrease in the electrical power applied to the superhard composite material.

Similarly, the method according to the invention may comprise at least two temperature cycles, with each of said temperature cycles including an increase and a decrease in the temperature applied to the superhard composite material.

The creation of such a binder-depleted area in the superhard composite material can be the final step in the method of processing the material for cutting, machining, or drilling tools. However, it may also be followed by a complementary processing such as a coating to further improve the mechanical, chemical, and/or thermal properties of the materials. Thus, the method according to the invention may comprise an additional coating step, with said coating preferably comprising the deposition of a nanocrystalline or microcrystalline diamond layer.

For example, the coating can be deposited by a PVD (Physical Vapor Deposition)- or CVD (Chemical Vapor Deposition)-like method, preferably by a CVD-like method.

According to a second aspect, the invention relates to a superhard composite material with a binder-depleted area 22 characterized in that the binder-depleted area 22 has a continuous gradient 221 of binder content extending over a depth greater than or equal to 500 µm.

Preferably, the continuous gradient 221 of binder content extends over a depth greater than or equal to 700 µm, more preferably greater than or equal to 1000 µm, and even more preferably greater than or equal to 5000 µm.

The continuous gradient 221 of binder content advantageously has a continuous profile and no levels. This can be reflected by the fact that, advantageously, the continuous gradient 221 of binder content has a linear correlation coefficient between the depth and the binder content greater than 0.7, more preferably greater than 0.8. The linear correlation coefficient of the continuous gradient 221 of binder content can be measured over a depth greater than or equal to 500 µm, preferably greater than or equal to 700 µm, and even more preferably greater than or equal to 1000 µm, for example greater than or equal 5000 µm.

The superhard composite material having a binder-depleted area 22 according to the invention also has an area 24 retaining the nominal binder content. In addition, the superhard composite material with a binder-depleted area 22 according to the invention does not include areas with a binding concentration significantly higher than the nominal binder concentration in the original superhard composite. Indeed, unlike some methods of the prior art, the manufacturing method of the invention does not concentrate the binder within the composite material. For example, the binder content in the superhard composite material with a binder-depleted area 22 is generally less than or equal to 120% of the nominal binder content in the original superhard composite. Thus, advantageously, the superhard composite material with a binder-depleted area 22 according to the invention does not include an area with a binder content greater than 120%, preferably greater than more than 110%, of the nominal binder content in the original superhard composite material.

The absence of such an area with a binder concentration significantly higher than the nominal binder concentration allows to obtain a material that is generally harder and also has good thermal shock resistance. Indeed, the areas with a binder concentration higher than the nominal binder concentration are generally less hard and can create weaknesses in the material.

The nominal binder content corresponds, within the meaning of the invention, to the binder concentration in the composite material before implementing the method according to the invention. This may also correspond to the most stable binder concentration in the superhard composite material, that is to say the one most frequently found in the superhard composite material according to the invention.

Thus, in a particular embodiment, the superhard composite material includes an area 24 retaining the nominal binder content and having a substantially constant binder concentration. For example, the area 24 with a substantially constant binder concentration may extend over several hundred micrometers and is preferably at least one millimeter deep, preferably at least two millimeters deep. In addition, the area 24 with a substantially constant binder concentration may have a binder content greater than 4 wt.% (by mass fraction), preferably greater than 6 wt.%.

In addition, the superhard composite material with a binder-depleted area 22 may comprise a cutting surface with a binder content of less than 3 wt.% (by mass fraction), preferably less than 2%, more preferably less than 1%. In one embodiment, the superhard composite material with a binder-depleted area 22 comprises a cutting surface with a non-zero (strictly greater than 0%) binder content.

The processing method and the processed material obtained are particularly useful for manufacturing superhard cutting parts or tools for cutting applications such as wood or metal machining, drills for deep drilling, and all applications where it is necessary to use superhard cutting tools. These tools or parts are particularly advantageous when used under severe conditions.

Thus, according to another aspect, the invention relates to a cutting, machining, or drilling tool comprising the superhard composite material, with a binder-depleted area 22, according to the invention. The invention also relates to a part comprising the superhard composite material, with a binder-depleted area 22, according to the invention. These parts can be, for example, dies, molds, or punches.

These tools or parts according to the invention have excellent resistance to wear, deterioration, and heat. Preferably, the superhard composite material is located at at least one cutting, machining, or drilling surface of said tool.

For example, the invention relates to a drilling tool (for example drill bit) with a head topped by at least one superhard composite material cutting-edge with a continuous gradient 221 of binder content according to the invention.

EXAMPLES

The following examples illustrate embodiments of the invention to which the invention is not limited.

1. Polycrystalline Diamond - Cobalt Composite Material Comprising a Depleted Area 1.1 Formation of a Binder-Depleted Area

The superhard composite material used for this example is a composite material based on polycrystalline diamond and a cobalt binder. Such a superhard composite material can be synthesized under high pressure and high temperature (from 1400° C., 5 GPa). The diamond particles are generally associated with a source rich in Co binder and subjected to high pressure. Heating to a temperature at least equal to the liquidus of Co under pressure allows the binder to diffuse, and after cooling and decompressing, the structure of the composite is formed. The Co binder allows to bind the diamond particles, but it is also a catalyst allowing to create bonds between the diamond grains.

An absorbent material was used and placed between the superhard composite material and the anode. The absorbent material used is made of niobium in the form of a plate, niobium in the form of a powder, or copper in the form of a plate.

The superhard composite material was subjected to an electric current of 60 Amperes, at a temperature of 500° C. and for a period of 90 minutes.

1.2 Binder Concentration in the Depleted Area

The binder concentration in the superhard composite material with a binder-depleted area was measured by energy-dispersive spectrometry and by wavelength-dispersive spectrometry.

FIGS. 3A to 3C show a graph of the cobalt distribution in the superhard composite material with a binder-depleted area as a function of the depth from the cutting surface when using a niobium plate (FIG. 3A), a niobium powder (FIG. 3B), or a copper plate (FIG. 3C), respectively.

FIG. 3A shows that in the presence of a niobium plate, the invention allows the formation, within a composite material comprising cobalt at about 7.5% of the mass fraction, of a binder-depleted area over a depth of more than 1 millimeter. The measurement uncertainty is estimated at ±1 m%. In addition, this depleted area has a continuous gradient of binder with a slope of about 6.12 m.mm⁻¹% (namely mass fraction per millimeter) over a depth of more than 700 µm.

FIG. 3B shows that in the presence of a niobium powder, the invention allows the formation, within a composite material comprising cobalt at about 7.5% of the mass fraction, of a binder-depleted area over a depth of almost 1 millimeter. The measurement uncertainty is estimated at ±1 m%. In addition, this depleted area has a continuous gradient of binder with a slope of about 3 m.mm⁻¹% over a depth of more than 500 µm.

FIG. 3C shows that in the presence of a copper plate, the invention allows the formation, within a composite material comprising cobalt at about 7.5% of the mass fraction, of a binder-depleted area over a depth of almost 1 millimeter. The measurement uncertainty is estimated at ±0.9 m%. In addition, this depleted area has a continuous gradient of binder with a slope of about 2.7 m.mm⁻¹% over a depth of more than 700 µm.

2. Polycrystalline Tungsten Carbide - Cobalt Composite Material Comprising a Depleted Area 2.1 Formation of a Binder-Depleted Area

The superhard composite material used for this example is a composite material based on tungsten carbide and a cobalt binder.

A niobium-based absorbent material was used in the form of a plate and placed between the superhard composite material and the anode.

The superhard composite material was subjected to an electric current of 520 Amperes, at a temperature of 700° C. and over a period of 50 minutes.

2.2 Binder Concentration in the Depleted Area

The binder concentration in the superhard composite material with a binder-depleted area was measured by wavelength-dispersive spectrometry.

FIG. 4 shows that in the presence of niobium, the invention allows the formation, within a composite material comprising cobalt at about 13% of the mass fraction, of a binder-depleted area over a depth of almost 10 millimeters. In addition, this depleted area has a continuous gradient of binder with a slope of about 0.1 m.mm⁻¹% over a depth of more than 8 millimeters.

These results show that the method according to the invention allows the superhard composite material to be depleted over very large depths in order to form a continuous gradient of binder. In addition, the temperatures used are tolerated by all polycrystalline microstructures usually used in superhard composite materials and the method does not require the use of polluting substances. 

1. A method of treating a superhard composite material (21) comprising a polycrystalline microstructure and a binder, said method comprising the following steps: contacting (200) a surface of said superhard composite material (21) with an absorbent material (30), and applying (300) an electric current to the superhard composite material (21), causing the binder to move from the superhard composite material (21) to the absorbent material (30) so as to create a continuous gradient (221) of binder content within the superhard composite material (21).
 2. The method according to claim 1 characterized in that the polycrystalline microstructure includes crystals selected from the crystals of: tungsten carbide, diamond, silicon carbide, silicon nitride, and boron nitride.
 3. The method according to one of claims 1 or 2 characterized in that the polycrystalline microstructure consists essentially of tungsten carbide crystals.
 4. The method according to any one of claims 1 to 3 characterized in that the binder includes at least one element selected from the following elements: iron, ruthenium, osmium, hassium, cobalt, rhodium, iridium, meitnerium, nickel, palladium, platinum, darmstadtium, molybdenum, and titanium.
 5. The method according to any one of claims 1 to 4 characterized in that the binder includes cobalt.
 6. The method according to any one of claims 1 to 5 characterized in that the absorbent material (30) is capable of interacting with the binder so as to form binary, ternary, or quaternary compounds.
 7. The method according to any one of claims 1 to 6 characterized in that the absorbent material (30) is provided on a surface adjacent to an area to be binder-depleted.
 8. The method according to any one of claims 1 to 7 characterized in that the electric current applied to the superhard composite material (21) has an electric current density between 0.1 and 50 A/mm².
 9. The method according to any one of claims 1 to 8 characterized in that the electric current is applied over a period of at least five minutes.
 10. The method according to any one of claims 1 to 9 characterized in that the electric current is applied to the superhard composite material (21) via electrodes positioned, on the one hand, on a surface adjacent to an area to be binder-depleted, and, on the other hand, to a surface opposite to the area to be binder-depleted.
 11. The method according to any one of claims 1 to 10, characterized in that it is carried out at a temperature lower than the melting temperature of the binder.
 12. A superhard composite material including a binder-depleted area (22) characterized in that the binder-depleted zone (22) has a continuous gradient (221) of binder content extending over a depth greater than or equal to 500 µm.
 13. A cutting, machining, or drilling tool comprising the superhard composite material according to claim
 12. 